A motor-driven power steering unit imparts a motor-generated auxiliary steering force to a steering force transmission system. This unit controls an electric current being supplied to the electric motor based upon steering information such as an angle through which a steering wheel is turned, the steering speed and the steering force or the speed of the vehicle. An auxiliary steering force generated from the motor as well as a manual steering force being applied to the wheel is allowed to act on the vehicle's wheels via a transmission means such as a steering gear mechanism so as to steer the vehicle's wheels. For a rack-and-pinion power steering unit, the manual steering force and the auxiliary steering force are allowed to act on the rack so as to move it from its neutral position to either of a rightward axial direction or a leftward axial direction. This motion of the rack is transmitted to vehicle's wheels via the transmission means such as the tie rod, thereby steering the vehicle's wheels.
In the motor-driven power steering unit generally described above, either the rightmost or leftmost position to which the rack is permitted to move (this position is hereunder referred to as a maximum steering angular position) is limited by bringing the rack into contact with a fixed reference part such as a gear-case. This is done in order to ensure that the rack will not move beyond its maximum steering angular position and so that the steering wheel will not be turned in a direction to cause the rack to be moved beyond its maximum steering angular position. Therefore, if the rack is at its maximum steering angular position (or stated more simply, when it is at the maximum steering angle), there is no need to impart the motor-generated auxiliary steering force to the rack. Rather, it is desired to limit the auxiliary steering force being produced by the motor so as to reduce its power consumption.
To meet this need, an improved motor-driven power steering unit has been proposed, as described above, in Japanese Utility Model Laid-Open Publication No. 193868/1985. This steering unit has a microswitch mounted in a gear-case for detecting the maximum steering angular position of the rack. The auxiliary steering force generated from the motor is decreased when the rack is found to be at its maximum steering angular position on the basis of an output from the microswitch.
However, the prior art motor-driven power steering unit described above has a disadvantage because a wire harness must be provided between the microswitch, in the gear-case, and the control unit which is very distant from the switch. Thus, the wiring needed to install the switch will be cluttered which adds to the complexity of the overall design of the steering unit. Furthermore, installing a microswitch, in the prior art motor-driven power steering unit, requires a considerable amount of precision in the position of installation. Thus, the number of steps involved in assembly operations is unavoidably increased which leads to a higher manufacturing cost.